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The field of the invention is CT scanners and more specifically volumetric CT scanners that facilitate rapid collection of data required for CT imaging.
Many different types of medical imaging systems have been developed that are used for different purposes. Perhaps the most common type of imaging system category includes X-ray systems wherein radiation is directed across a portion of a patient to be imaged and toward a detector panel. An exemplary X-ray detector panel includes a Csl scintillator coupled with an amorphous silicon array. With radiation directed toward a region of a patient to be images (i.e., a region of interest), the region of interest blocks some of the radiation and some of the radiation passes through the region and is collected by the panel. The amount of radiation that passes through the region along the trajectory of a given radiation ray depends upon the type of tissue along the trajectory. Thus, a tumor may block more radiation than flesh and bone may block more radiation than a tumor and so on. Hence X-ray system can be used to collect a xe2x80x9cprojectionxe2x80x9d through a patient. The aforementioned detector panels are generally referred to hereinafter as digital detector panels.
Another imaging system type is generally referred to as a computerized tomography (CT) system. An exemplary CT system includes a radiation point source and a radiation detector mounted on opposite sides of an imaging area. The point source generates radiation that is collimated into a fan beam including a plurality of radiation rays directed along trajectories generally across the imaging area. A region of interest is positioned within the imaging area. With the radiation source turned on the region of interest blocks some of the radiation and some of the radiation passes through the region and is collected by the detector. As in X-ray systems, the amount of radiation that passes through the region of interest along the trajectory of a given radiation ray is dependent upon the type of tissue along the trajectory.
In CT systems the source and detector are rotated about the region of interest so that radiation xe2x80x9cprojectionsxe2x80x9d through the region can be collected for a large number of angles about the region. By combining the projections corresponding to a volume through the region of interest using a filtering and back projecting technique, a three-dimensional tomographic image of the region volume is generated.
Several factors have to be considered when determining the best way to configure a CT imaging system including relative system costs and resulting image quality.
Referring to FIG. 2, an exemplary CT source 14 and detector 18 are illustrated as being positioned on opposite sides of an imaging area 21. The source 14 is collimated to form beam 16 having a plurality of rays (not separately numbered). For a typical human torso 22 exam a 50 cm field-of-view (FOV) is required. In any CT system the geometry of the system causes a magnification factor (defined as the ratio of the source to detector distance over the source to isocenter (ISO) 24 distance) such that the dimension of the detector array 18 across the fan beam must be greater than the FOV at the region of interest position. In an exemplary CT system the magnification factor is approximately 1.7 so that the minimum detector panel width is 85 cm as illustrated.
One way to construct CT detectors is to configure a large number of CT detector elements (not separately numbered) in an arc about the radiation source 14 as illustrated in FIG. 2. An exemplary detector 18 may include as many as 8 rows of elements perpendicular to a translation or Z-axis where each row may include several hundred elements along the fan beam width (i.e., along the 85 cm width as illustrated in FIG. 2). In addition to the detector elements themselves acquisition circuitry is provided for each detector element for changing an element generated signal into a digital signal for processing. Advantageously such elements can be constructed and configured such that essentially no gap exists between adjacent elements and therefore data that can be used to generate diagnostically useful images can easily be collected.
One problem with CT detectors constructed as described above is that the overall configurations are extremely expensive due to the number of elements and corresponding acquisition circuitry. In addition the structure that maintains element positions with respect to the source is often relatively complex.
One solution to overcoming the problems associated with detectors requiring huge numbers of detector elements and corresponding acquisition circuitry is to provide a single silicon wafer based detector like the digital detectors described above. Thus, one digital detector having a width of 85 cm could be used to collect all CT acquisition data thereby avoiding the expense of separate elements and acquisition circuits.
While digital detectors are extremely useful, unfortunately the silicon wafers or panels required to construct such detectors are only mass produced with relatively small length and width dimensions. The wafer dimensions dictate the size of the detector and hence the FOV. Thus, there is no mass produced digital detector panel that has a width of 85 cm. While large silicon wafers could be produced for such panels ability to achieve consistent manufacturing quality of such large wafers is questionable and cost associated with such an effort is prohibitive.
One solution is to configure a detector panel that collects data across less than an entire FOV and rotate the source and panel through more than 180 degrees about the region of interest. For example, a detector panel having a width dimension slightly greater than one half the full FOV may be configured. For such a panel 360 degrees of rotation would be required to collect data to produce an artifact free image.
While a half FOV panel would be less expensive than a full FOV panel, again, such panels are relatively large, cannot be configured using mass produced silicon wafers and hence are still relatively expensive.
An exemplary embodiment of the invention includes an apparatus including a large CT detector comprised of a plurality of smaller digital detector panels wherein the panels are sized and arranged in side by side fashion to extend across a fan beam and so that conjugate rays generated during data acquisition always include at least one ray that subtends a detector panel and generates a collected signal even where the other ray in the conjugate pair is directed at a gap between panels.
The invention also includes a method to be used with the aforementioned detector, the method including, after data is acquired, interpolating across each gap to generate a modified data set and then combining the modified data set including the data interpolated across the gaps with collected signals corresponding to rays that are conjugates of the rays directed at the gaps to generate back projection data for each of the gap directed rays.
These and other aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefor, to the claims herein for interpreting the scope of the invention.